Highlights• Global activation energy depends on conversion in pyrolysis of all polymers.• Activation energy variability is proportional to chemical complexity of polymers.• An Evans-Polanyi relationship for initiation reactions of polymer pyrolysis is proposed. AbstractRecent advances in plastics recycling confirm the high potential of pyrolysis technologies to enhance recovery and selectivity rates. Modelling the kinetics of this complex process to stimulate scaling-up opportunities remains a major challenge.This study is an attempt to develop practical quantitative reactivity indices for the pyrolysis of natural and synthetic polymers. Representative samples from both categories (coal and pine vs. PET, PE and PMMA) were selected. Their weight loss during pyrolysis was determined experimentally and analyzed using judicious lumping procedures for its initiation, propagation and termination steps. The combined experimental and theoretical approach allowed us to determine apparent activation energies using the isoconversional Friedman method; and the use of Benson's group contribution method generated reaction enthalpies for the initiation reactions. Increasing activation energies with conversion in each case indicated that bond scission proceeds in order of increasing bond strength. The greater chemical complexity of natural polymers was reflected in the higher coefficients of variability for activation energy and wider ranges of reaction enthalpies. A structure-reactivity relationship based on the Evans-Polanyi theory was used to test the hypothesis that primary pyrolysis kinetics is controlled by cleavage of weakest chemical bonds. While the results show that this approach is promising, especially if confirmed by extension to a larger data set, they also suggest the need to compare the relative kinetic importance of initiation and propagation steps in the sequence of polymer pyrolysis reactions.